1. Field of the Invention
The invention pertains to the field of reflective displays and methods of manufacture. More particularly, the invention pertains to electrophoretic displays containing barrier walls and barrier electrodes, and fabricating such displays using fibers and/or hollow tubes.
2. Description of the Related Art
There are several different methods of producing a reflective display, however when fabricating very large, energy efficient flat panel displays the most promising method uses an electrophoretic switching material. Early work such as that described in U.S. Pat. No. 3,767,392, xe2x80x9cElectrophoretic Light Image Reproduction Processxe2x80x9d, used a suspension of small charged particles in a liquid solution (electrophoretic suspension) for displaying a light image. The suspension is sandwiched between two glass plates with electrodes on the glass plates. If the particles have the same density as the liquid solution then they will not be effected by gravity, therefore the only way to move the particles is by using an electric field. By applying a potential to the electrodes, the charged particles are forced to move in the suspension to one of the contacts. The opposite charge moves the particles to the other contact. Once the particles are moved to one of the contacts, they reside at that point until they are moved by another electric field, therefore the particles are bistable. The electrophoretic suspension is designed such that the particles are a different color than the liquid solution. Therefore, moving the particles from one surface to the other will change the color of the display.
It is very difficult to address most electrophoretic displays. Since electrophoretic materials do not have a voltage threshold, displays fabricated with the electrophoretic materials have to be individually addressed at each pixel. One method to combat this problem is to use active devices that have a voltage threshold. There are two different types of active devices. One type is a transistor array similar to that used in an active matrix liquid crystal display. The second type is a plasma similar to that disclosed in U.S. patent application Ser. No. 09/517,759, entitled REFLECTIVE ELECTRO-OPTIC FIBER-BASED DISPLAYS, hereby incorporated herein by reference.
Active devices are complicated and expensive to fabricate and are usually limited in size. Therefore, an addressing scheme where the display can be passively addressed is desired. One such addressing scheme was introduced by Philips in U.S. Pat. No. 4,203,106 where they added a third control electrode to create a voltage threshold to manage the migration of particles. This third electrode is patterned with holes and is placed over and orthogonal to the attraction electrode. Controlling the voltage on the control electrode causes the particles to migrate into the holes in the control electrode, in turn, changing the color of the display.
Another passively addressed display was invented at Copytele, U.S. Pat. No. 5,345,251. This display is constructed using interleaved electrodes and an orthogonal electrode. The movement of the particles is in the plane between the interleaved electrode and is controlled by the orthogonal electrode. The addressing electrode controls the movement of the particles in all of these passively addressed displays. Since the particles do not have a voltage threshold, it makes it very difficult to matrix address the display. In order to achieve passive matrix addressing, a barrier must be added to the cell between the two driving electrodes.
A display which uses a barrier between drive electrodes was disclosed by E. Kishi, et al., xe2x80x9cDevelopment of In-Plane EPDxe2x80x9d, SID 00 Digest, pp. 24-27. Two types of barriers were disclosed: a physical barrier 48, shown in FIG. 1, and an electrical barrier 44, shown in FIG. 2. Both displays are constructed by building up the structure on a top 30B and a bottom 30T substrate. A separator 45 is used to create the cell that houses the electrophoretic material 37. The drive electrodes 43 and 42 are electrically isolated from the cell and each other using dielectric layers 46 and 47. The operation of these displays is achieved by placing voltages on both driving electrodes 43 and 42 and controlling the flow of particles over the barriers using the control electrode 41.
Assuming the particles 37 are positively charged, then the display is in a holding state when a large positive voltage is applied to the control electrode 41, a small positive voltage is applied to the 1st driving electrode 43, and a negative voltage is applied to the 2nd driving electrode 42. In the case where the barrier is created by an electric field (FIG. 2), a positive voltage is applied to the barrier electrode 44. To move the particles 37 from the 1st driving electrode 43 to the 2nd driving electrode 42, the positive voltage on the control electrode 41 is reduced. In this case, the particles 37, which are repelled from the 1st driving electrode 43, are allowed to flow over the barrier to the 2nd driving electrode 42.
This passive method of addressing by adding barriers helps in addressing the pixel, but has problems addressing more than one row in a display. In addition, the display will have a high manufacturing cost because of the multiple steps needed to create the structure and pattern the electrodes in the display. The display will also be limited in size since the structure is built-up on a substrate. The following invention solves the manufacturing and addressing issues and is cost effective in a large panel display.
The invention includes the use of hollow tubes filled with an electrophoretic material sandwiched between two plates to form a reflective display. The hollow tubes have either barrier walls or an electrostatic barrier, which restricts the flow of electrophoretic particles within the hollow tubes. The flow of electrophoretic particles over these barriers is controlled using electric fields, which makes it possible to matrix address the electrophoretic displays. Wire electrodes built into the hollow tubes and electrodes on the two plates are used to create the electric field and address the display. The electrodes on the plates can be replaced with wire electrodes or wire electrodes contained within a fiber. The plates are preferably composed of glass, glass-ceramic, polymer/plastic or metal, while the hollow tubes are preferably composed of glass, polymer/plastic or a combination of glass and polymer/plastic. In addition, color is optionally imparted into the display using colored tubes, adding a color coating to the surface of the tubes, or adding the color to the electrophoretic material. Reflectivity within the display is accomplished by using a reflective material to fabricate the tubes, coating the tubes with a reflective material or coating one of the two plates with a reflective material. The display can also function in a transmissive mode by applying an illuminating back to the display.